1. Field of the Invention
The present invention concerns a method of producing reduced iron pellets formed by reducing iron oxide pellets containing Zn ingredients. More in particular, the invention relates to a method of producing reduced iron pellets formed by reducing iron oxide pellets containing dusts (including sludges) containing Zn ingredients formed in iron works.
2. Description of the Related Art
In iron and steel making processes including blast furnaces, converters and electric furnaces, various kinds of iron oxide-containing dusts or sludges are formed in a great amount which are recovered and reused as iron sources (used in the form of pellets or sintered ores). However, iron oxide-containing dusts or sludges resulting from iron and steel making processes may contain noxious Zn ingredients such as Zn oxide, to result in a problem that iron sources of high Zn amount can not be used as the starting material (hereinafter, used as the meaning of feed material) for blast furnaces. In the blast furnace, if the starting material contains a great amount of Zn, the operability of the blast furnace is worsened, as well as it leads to a problem that Zn attacks refractories of furnace walls.
As described above, use of dusts or sludges containing Zn-containing iron oxide has been restricted and, particularly, iron oxide-containing dusts or sludges containing Zn ingredients formed in a great amount from blast furnaces have not been effectively recovered and reused as iron sources at present.
In view of the above, methods of producing reduced iron by removing Zn from pellets which are molded from iron oxide-containing dusts or sludges resulting from iron and steel making processes have been proposed in recent years. There can be mentioned, for example, a method of using a rotary kiln furnace (refer to Japanese Patent Examined Publications Sho 51-13083 and 55-21810) and a method of using a rotary hearth furnace (refer to Japanese Patent Unexamined Application Hei 5-125454 filed in Japan corresponding to U.S. Pat. No. 5,186,741).
The method of using a rotary kiln has a merit in that Zn can be decreased to as low as 0.01 mass % and metallization ratio can be improved to about 80% (about 87% as a reducing ratio). However, since pellets are tumbled and baked in a rotary kiln in this method, results in a problem that pellets are powderized during operation, which are melted and deposited to form kiln rings, thereby making the operation impossible. Furthermore, since the total amount of Zn in the starting material fed to the blast furnace is controlled (for example, 0.2 kg Zn/t-pig iron), and the amount of using reduced pellets containing Zn is restricted, the amount of Zn has to be further decreased for use as the starting material for the blast furnace also in this method.
Than, in a method of using a rotary hearth furnace described in Japanese Patent Unexamined Application Hei 5-125454, reduced pellets are produced by forming green pellets comprising a mixture of dusts from steel works, carbonaceous material such as coal or coke and an organic binder, feeding the green pellets on a layer of pellets baked in a rotary hearth furnace, drying them at a temperature lower than 900.degree. C. for 10 to 15 min, thereby forming coked dried pellets and then reducing the pellets at a temperature lower than 1150.degree. C. for 20 to 30 min. While Japanese Patent Unexamined Application Hei 5-125454 describes for the reduced pellets that iron oxide is reduced to a metallic state and Zn ingredient is removed from the reduced pellets, it does not teach about the actual extent of reduction ratio and the amount of Zn. Furthermore, the method does not define the contents of iron and Zn in the pellets and the amount of carbonaceous material to be added. As can be seen from the description that the reduced pellets, when discharged from a rotary hearth furnace at about 1000.degree. C., may possibly include a considerable amount of carbon (as high as 12 mass %) when they are discharged from the rotary hearth furnace), it is suggested that a great amount of carbon (as much as 12% by weight) may remain after the reduction in this method. Residue of a great amount of carbon brings about a problem that not only the content of iron in the reduced pellets is decreased to lower the utilizing efficiency as an iron source, but also the strength of the reduced pellets per se is deteriorated. Particularly, when coal is added as the carbonaceous material, it tends to greatly deteriorate the strength of the reduced pellets per se. If the strength of the pellets is lower, it results in a problem that the reduced pellets are pulverized or crushed in the blast furnace, to lower the air ventilation in the blast furnace and worsen the blast furnace operation, so that the pellets can not be used as the starting material for the blast furnace.